Alright, let's dive right on into the good stuff. The Warthog will drop you off. Race over to the dead Grunt and pick up a 100% Plasma Pistol and a single Plasma Grenade. Assuming you're facing the chasm, turn left and walk a few meters until you cross over a Battle Rifle. Pick it up. Walk up the ramp and flip the switch to trigger the blast doors. As they are opening, commandeer the Warthog and drive it inside once you are able. Park it as shown in the video below. This will help prevent our Marines (who seem to constantly drive under the influence), from taking control of the driver's seat. Hop up on the catwalk, arm yourself with the Plasma Grenade, charge into the next room, and toss it at the nearest Jackal. Don't wait to see results. Quickly flip the switch on the upper walkway and drop back down with your Marines. Be prepared. The second those doors open, the surviving Grunts will start chucking tons of grenades in your direction. Make them your top priority with the Battle Rifle and let your Marines deal with the remaining Jackals.
If you start running low on ammo, there's a Battle Rifle in the next room just to the left of the large blast door near all the debris. I recommend using that as your first ammo source, because as soon as the room is initially cleared, three Grunts will emerge from the side door leading to the next area. If you aren't around to stop them, they'll pwn your Marines with Plasma Grenades.
Once they're dead, back track to the upper walkway of the previous room, past the glass, and into the small backroom where you'll be able to grab addition ammo and snag any extra grenades you may need.
Fetch the Warthog and position it in the manner shown in the first video. Here's hoping you have a few Marines left alive! Flip the switch to open the next blast door, and take cover near the Hog.
Let the fun begin! A grenade war between the Marines in the Grunts should now be in full swing. Target the Grunts with your BR and take them out as quickly as possible. The quicker they die, the more helpful your Marines will be at mopping up the remaining troops. Towards the end of the battle, one or both Ghosts will gain a driver. If your Marines are still active, they'll spam the hell out of both of them and save you the trouble of having to engage. In short, wait it out. For a more visual look on how to tackle this entire beginning section, have a look at the video below.

Part - 2:

Ready to tackle your first Anti-Air Wraith? Saddle up in the Warthog with a gunner and passenger, flip the switch, and drive out into the open air. Follow the path on the far left wall and try to run down the Brute Captain. This will typically cause at least a few of the Grunts to kamikaze and blow themselves to pieces. Saves you the ammo.
Park the Hog behind the column as shown in the below video. Where you park it is important. If you park it too close to the dock, when you try to engage any of the enemies on the shack's roof, the Ghosts from the lakebed will flank you and deal some heavy damage. Park it right where you see in the video. Next, hop out and try to kill the Brute just under the Phantom without getting too close. Don't waste more than two shots with the Plasma Pistol, however. If you wait too long, you'll miss your opportunity to jack the Wraith that is about to be dropped on the opposite platform. If you can't tag him in more than two shots, keep moving. But if you can, you'll help yourself out later.
On to the Wraith. At this point you should be hugging the left wall and hauling ass to get to the exit door. See that Traxis box near the door? Hide inside. A Phantom will drop off a Mortar Wraith. Jack the back and bash once for shields. Wait for the Phantom to leave, then jump on top to combo-kill the secondary gunner and take his Bubble Shield.
Here's where the dock battle starts to matter. It can play out one of two ways. Either your Marines will win and will assist in causing a distraction for the Ghosts, or the Covenant will win, and you'll now have a small platoon of enemies making there way over from the dock to your current location. If you killed the Brute, then you drastically increased your Marines' chances of success. Otherwise, be prepared to pick off the remaining Covie troops one by one as they climb the ledge to get you. Don't worry about disabling the Wraith entirely until you've killed the infantry. As long as you stay near it, and there is no secondary gunner, then it won't bother you. When it's just you and the Wraith, jack the front and bash it five times to kill the driver. The Wraith should still be drivable at this point. Take it and use it to clear the rest of the area. Below is a video demonstrating how to get to this point.

Target the Ghosts first. After you've killed one, you'll have a few seconds before a Phantom shows up to drop off two more. I suggest hopping out of the Wraith and taking cover behind it until the Phantom leaves. Proceed in destroying the Ghosts. Before you take on the AA Wraith, truck it over to the structure on the end of the dock to replenish your BR ammo. Now use the Wraith to destroy the AA Wraith. You should be near the exit door when you do this so you can avoid the Banshees that are about to spawn.
The doors will open, and you'll see a dozen Grunts heaving Plasma Grenades towards the other side of the room. I recommend taking them out with the BR. That goes double for the one toting a Plasma Cannon. A Brute Captain will also assault your position. Combo-kill him like you would any other. He merely has a Spiker, so nothing fancy. You can use the Wraith to kill the Jackals if you wish to save ammo.
Here's where things might get a little tricky. Peek inside the room. Take a gander at the far right-hand corner. Normally there will be a small trailer toting a medium-sized crate. If it's still intact, then you can rush over, drop a Bubble Shield for backup if you have one, jump on top of the crate, onto the catwalk, and escape into the next room. If it has been destroyed, you can take the Warthog use it in place of the crate. I recommend shielding the Hog with your Bubble Shield, so you might want to drop that before driving the Hog into the room. If you did it correctly, you just avoided a small, yet complicated skirmish against a Plasma Cannon-wielding War Chieftain and his minions. An ideal scenario should play out like this.

Part - 3:

We will be handling the next area in a radically different way than we did in 1.0, so proper preparation is necessary. You need to know offhand that we'll be assaulting the Scarab directly. The best tools for this are your Battle Rifle, Plasma Pistol, and a full stock of both Plasma Grenades and Spike Grenades. Stock up on BR ammo from the ammo case just behind the Mongooses, then hop onboard and drive straight through the lakebed and into the small building in the middle like so...

Let your Marines fight it out from here until the Scarab arrives. In most cases, they'll be able to take out the AA Wraiths, Ghosts, and Choppers by themselves. It's the only part in the game where I can almost rely on them entirely to do something for me. You can pitch in if you want, but just be wyary of your grenade count and don't step outside unless the immediate area is clear. It doesn't take too many shots for a Ghost to bring you from full health down to zero.
When the battle starts winding down, headshot the Marine on the back of the Mongoose wielding the Rocket Launcher. Also, pick it up and put it back down to prevent it from despawning. You'll need it later.
Right before the Scarab arrives, rip the Missile Pods from their bases and discard them. This will help to prevent you from becoming a victim of Devil Dog Wrath.

Gawd, I hate those Marines...

Anyway, once the Scarab is on its way, scale the top of the left crane and wait for it to drop. Use your BR to start picking off Grunts. The Scarab will move around towards the left side of the crane. Take this opportunity to jump onboard. Be aware that there are typically two Brute Minors with Spikers and one Brute Captain wielding a Brute Shot onboard along with a small squad of Grunts. Now, you can handle this one of two ways. The first is to do what I did in the following video...

Jump onboard and take the left hand side. Be ready to plasma-charge the first Brute you see. Quickly follow up with your BR to bring it down. Take them out one at a time via this method. If you run out of Plasma Pistol ammo, use your sticky grenades. You can even knock them off the Scarab entirely with a well placed Frag Grenade. Otherwise, you can hide in the small area directly below the Anti-Air gun and wait for them to eventually fall off. I wouldn't vote for the latter since a lot can go wrong the longer you wait. Bash the Scarab's core once you've killed the crew, jump off, and RUN!

Part - 4:

If you still have Plasma Pistol ammo left, then drop the BR for the Rocket Launcher you "stole" earlier and take it with you into the next area. Don't worry, there are plenty more BR's where we're going. Collect one of them and temporarily drop your Plasma Pistol. By the way, make sure Arbiter follows you into the area. He may need to be coerced, so do whatever is necessary. Use this method to pass along your Rocket Launcher to the Arbiter.

Go back and get your Plasma Pistol, and then proceed forward. Arbiter will assault the Brutes with the Rocket Launcher. When their shields are down, you can take the credit by landing the headshot. Feel free to spend ammo here. There's plenty of it. Move to the second floor once the first wave has been put down and move forward. This will trigger Arbiter and the Marines to do the same from below. Assist them from on high as they engage the enemy. When only a couple of Brutes remain, they will attempt to retreat. Let them for now. Again move forward. I like to jump on top of the crate like so to coerce the Arbiter into engaging the Brutes. I might also land a plasma charge or two.

Proceed forward once the area is clear, and here come the Hunters. While the Arbiter is usually pretty good about dealing with them on his own via the Rocket Launcher, you can pitch in by using the Hunters' peculiar weakness against them. Believe it or not, they seem to have a strange vulnerability to the Spiker. In short, it stuns them. So assuming you no longer have any use for your Plasma Pistol, go ahead and trade it out for a Spiker. An example of some serious Spiker pwnage can be viewed here...

See? You can haz credit for Arbiter's kills.

Part - 4:

The only thing standing between you and crippling the Covenant's Anti-Air defenses is a Weapon Chieftain and his minions of Brutes, Grunts, and Jackals. Watch out. Those two Captains have Carbines. You might try either pop-shooting or clip-shooting the Captains just to make your life easier. However, if they've already retreated, focus on taking out the closest units first; usually a few Grunts and a pair of measly Jackals. Assuming you're now an expert on Part-1, this shouldn't be anything new to you. After you venture far enough out into the open, the Arbiter will join you with his UBER ROCKETS OF DEATH! Let him and the grenade-spamming Marines take it from there. While a few more Brute Minors will join the retreated Covenant forces, it won't be an issue for those high-explosive rockets. Just for fun, watch the video...

When all is quiet, grab a pair of Plasma Pistols and truck it to the AA Gun. When you get there, Grunts will start emerging from behind the boulders. Head-shot them as they scurry around. Don't get too close though. See that War Chieftain up near the gun? He's armed with a Fuel Rod Cannon and he knows how to use it!
This part is simple. Use the range-shot technique to blast away the Chieftains shields. Switch to your BR and pluck him a few times. It won't kill him, but this will trigger both him and his troops to retreat to the area under the AA gun. This means they won't bother you anymore.
Now for the Gun itself. Believe it or not, its core is extremely vulnerable to overcharge shots from the Plasma Pistol. Time them correctly as shown in the video below, and you can have the AA gun down with a mere TWO plasma charges.